Arrangement for converting direct current into alternating current



June 4, 1940. LElFER 2,203,467

ARRANGEMENT FOR CONVERTING DIRECT CURRENT INTO ALTERNATING CURRENT FiledApril 19, 1938.

INVENTOR ALFRED LEIFER ATTORNEY Patented June 4, 1940 UNITED STATESPATENT OFFICE ARRANGEMENT FOR CONVERTING DIRECT CURRENT INTO ALTERNATINGCURRENT Application April 19, In Germany Claims.

To avoid opening sparks when interrupting a direct current circuit it isalready known to shunt a condenser or a series connection of a condenserand ohmic resistance to the switch or to the load. 5 When the condenseris placed in parallel to the load the voltage of the load can onlydecrease in a'slow fashion, so that during the moment of opening no highvoltage can appear at the switch and therefore no appreciable spark. Inorder to obtain a possibly complete eiTect it would be advisable tochoose a large capacity and on the other hand a possibly low resistance.But this condition entails the disadvantage that a strong closing sparkappears since at the moment in which the switch is closed a chargingcurrent flows into the condenser which at a low resistance will be verylarge at the first moment. Therefore, a mean value must be chosen forthe resistor in order to prevent too large a spark at closing as well asat opening of the switch.

The difficulties due to the appearance of sparks are especiallyconsiderable in vibrating direct current converters (choppers) in whichthe voltage of' a direct current source is chopped and the polarityperiodically reversed and applied to the load. The weakening of theswitch spark in such vibrating direct current converters is, when usingthe said kn'own arrangement, not appreciable especially if the load ofthe direct current converter fluctuates.

In accordance with the invention, in order to weaken the switching sparkan electric relaxation oscillation generator is provided operating insynchronism with the switching and which applies to the load connectedto the direct current source the electrical energy liberated at themoment in which the switch is opened, this energy being applied in sucha manner that the current impulses and voltage impulses appearing at theopening of the switch will be compensated.

More especially, a series connection of an ohmic resistor and condenseris placed in parallelto the load on the one hand, and anauto-transformer on the other hand whose tap is connected through a glowtube to the point connecting the resistor with the condenser.

The advantage of the arrangement resides in that the balancing energycompensating for instance too rapid'a decrease of the voltage of theload at the moment of openingof the switch, can be supplied atsuflicient speed in view of the relaxation performance while a storingof energy can take place at such slow speed that no appreciableswitching spark can appear at closing of the switch.

1938, Serial No. 202,891

April 21, 1937 An example embodying the idea of the invention is shownin Figure l in connection with a vibrating direct current converter.

The voltage of the direct potential source E is applied across thedouble pole switch U and with alternating polarity to the line leadingto the output terminals A, B between which the load, not shown in thedrawing, is placed, said double pole switch being actuated for instanceby means of a vibrating spring such that always one pair of oppositecontacts will be closed and the other pair opened. There is placedbetween the terminals A and B, hence, in parallel to the load, on theone hand, a series connection comprising the resistor R and thecondenser C and on the other hand, a choke coil L acting asauto-transformer and having a center tap. The point connecting theresistor and condenser to each other is connected through a glow tube Gto the center tap of the coil. This tube acts as relaxation oscillatorin conjunction withtlie' direct potential source E, resistor R andcondenser C. The natural frequency of the relaxation circuit is chosenpreferably slightly lower than the double frequency of the vibratingdirect current converter. In the ,1 present arrangement, the relaxationoscillation is automatically synchronized by the interruption frequencyas will be seen from the following:

' During the time of closing, i. e., as long as the voltage of thesource E of, for instance, 100 volts is applied between A and B, thecondenser C will be charged across the resistor R to a large percentageof this voltage thus, for example, to 95 volts, so that the ignitionvoltage of the glow tube G, which may be, for instance, equal to 50volts, will almost be reached since the center tap of the transformer Thas 50 volts. Now, if the switch is opened the voltage between A and Bbegins to collapse immediately due to the action of the self-inductanceL and due to the load. But as soon as it has decreased in the presentcase to volts the ignition voltage of the glow tube is attained sincethe voltage of the center tap has fallen to 45 volts while the voltageof the condenser has remained at volts. The condenser now passes acrossthe glow tube a. current into the lower partial winding of the coil L,said current rapidly' reaching a maximum value whereafter it decreasesslowly. As a result thereof, the voltage at the center tap not onlyceases next to drop still further, but is at least maintained at itsvalue or since the operating voltage of the glow tube is somewhat lowerthan the ignition voltage, the voltage at the center tap even increasesslightly again at the first moment. The current impulse in the lowerpartial winding of the auto-transformer L now produces, however, throughinduction an additional voltage also at the terminals A and B whichcompensates the voltage drop thereby quenching the opening spark. Theinduced voltage will become zero only when the current has reached itsmaximum value, and will reverse its direction when the currentdecreases. The voltage between the terminals A and B will in this way begradually discharged or reversed. Accordingly, also the voltages at thecondenser and at the center tap decrease gradually until the glow tubeis extinguished. When the voltage is applied again in the oppositedirection no appreciable current impulse can be produced even if thecharge of the line A, B has not been reversed exactly to the propervoltage, because the condenser is disconnected from the circuit and itscharging must take place across the resistor R.

Eventually, it may be advisable to place a suitably dimensioned resistorin series to the glow tube so as suitably to fix the magnitude of thecurrent flowing at the moment of ignition of the glow tube and suit-ableto establish its speed of change.

The tap at the coil need not be exactly in the I center of the winding,and may suitably be so set that at opening of the switch, thevoltage-jump will be compensated as effectively as possible.

The choke coil may be the primary winding of a transformer serving fortransmitting the alter-, nating voltage.

I claim:

1. A converter device of the mechanical type for converting directcurrent into alternating current, comprising a source of direct current,an output load, switching means adapted to alternately connect thepositive and negative terminals of the source to the opposite terminalsof the load, a series connection of a resistance and a condenserconnected in shunt to the load, a glow tube connected between the commonterminal of said condenser and resistance and an intermediate point onthe load, the arrangement being such that with the opening of the switchmeans and the incident decrease of thevoltage applied to the load, thedifference between the condenser voltage and a fixed percentage of theload voltage is so impressed upon the glow tube that the same will beignited.

2. A converter device of the mechanical type for converting directcurrent into alternating current, comprising a source of direct current,a pair of output terminals for said device, switching means adapted toalternately connect the positive and negative terminals of the source tothe opposite terminals of the load, an inductance connected between theoutput terminals, a series connection of a resistance and a condensershunted across said inductance, and a glow tube connected between themid-tap of the inductance and the common terminal of the resistance andcondenser.

3. A converter device according to claim 2, wherein the tappedinductance serves as the primary winding of an output transformer.

4. In a converter device of the mechanical type for converting directcurrent into alternating current, a source of direct current, a load,means including a switching device arranged to be opened and closed forintermittently connecting said source to the load, said switching devicebeing subject to arcing when opened, a condenser interposed between thesource and the load for storing up energy from said source while saidswitching means is closed, a circuit including a gaseous discharge path,the switching device and said source, for discharging said energysubsequent to the opening of said switching means, said discharge beingarranged to produce a pulse of current through said source and theswitching means in opposite sense to the flow of current therethroughprior to said opening of the switching device to thereby reduce saidarcing.

5. In a converter system for converting direct current into alternatingcurrent, said system incorporating a mechanical make and break switchingmeans for intermittently connecting a source of dire-ct current to aload, said switching means being subject to arcing on break duringoperation thereof, the steps in a method of reducing the in-

